usb.py

import math


def solve(m: int, n: int, c2: float, c5: float) -> float:
    """
    Adapter that costs c2, provides 2 ports but takes up 1 port, so net gain
    is 1 port per adapter. Similarly, adapter that costs c5, provides 5 ports
    but takes up 1 port, so net gain is 4 ports per adapter.
    """

    devices, ports = m, n

    assert ports > 0
    assert devices >= 0
    assert c2 >= 0
    assert c5 >= 0

    if ports >= devices:
        return 0

    delta = devices - ports

    if c2 <= c5 / 4:
        return delta * c2

    if c5 <= c2:
        return math.ceil(delta / 4) * c5

    base = (delta // 4) * c5
    remainder = delta % 4
    return base + min(remainder * c2, c5)


def test():
    # ports >= devices
    assert solve(1, 1, 1, 1) == 0
    assert solve(1, 2, 1, 1) == 0

    # c2 <= c5 / 4
    assert solve(10, 1, 0.25, 1) == 2.25
    assert solve(10, 1, 0.20, 1) == 1.8

    # c5 <= c2
    assert solve(10, 1, 1, 1) == 3
    assert solve(10, 1, 1, 0.9) == 2.7

    # c5 more cost effective per port
    assert solve(9, 1, 1, 2) == 4
    assert solve(10, 1, 1, 2) == 5
    assert solve(11, 1, 1, 2) == 6
    assert solve(12, 1, 1, 2) == 6


if __name__ == "__main__":
    test()